Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ (1,332)
Accumulated other comprehensive loss at end of period	(1,161)		(1,332)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(148,938)		(168,598)	(145,034)
Tax benefit	53,761		62,229	55,133
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,332)
Losses reclassified from net accumulated other comprehensive loss	171
Net current period other comprehensive income (loss)	171
Accumulated other comprehensive loss at end of period	(1,161)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	268
Tax benefit	(97)
Total, net of tax	171
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	$ 268	[1]

[1] Included in interest charges.